Electrochemical devices, such as batteries and fuel cells, typically incorporate an electrolyte source to provide the anions or cations necessary to produce an electrochemical reaction. Batteries and fuel cells operate on electrochemical reaction of metal/intercalation compounds, metal/air, metal/halide, metal/hydride, hydrogen/air, or other materials capable of electrochemical reaction.
Metal/air batteries, or metal/oxygen batteries, with aqueous and non-aqueous electrolytes have attracted the interest of the battery industry for many years. Zinc-air batteries with aqueous alkaline electrolytes have been used successfully for hearing aids and other markets (including military applications) which require batteries with high specific capacity. The unique property of metal/oxygen batteries compared to other batteries is that the cathode active material, oxygen, is not stored in the battery. When the battery is exposed to the environment, oxygen enters the cell through the oxygen diffusion membrane and porous air electrode and is reduced at the surface of the catalytic air electrode, forming peroxide ions and/or oxide ions in non-aqueous electrolytes or hydroxide anions in aqueous electrolytes. When the anode is lithium and non-aqueous electrolyte is used, these peroxide and/or oxide anions react with cationic species in the electrolyte and form lithium peroxide (Li2O2) or lithium oxide (Li2O). The ratio of lithium peroxide to lithium oxide formed in Li/air batteries depends on several factors, such as catalyst, electrolyte selection, oxygen partial pressures.
The metal anode in metal/oxygen batteries has been studied and developed based on Fe, Zn, Al, Mg, Ca, and Li. It has been shown that metal/air batteries have much higher specific energy than that achieved by lithium metal oxide/graphite batteries. Lithium/oxygen batteries are especially attractive because the Li/O2 redox couple has the highest specific energy among all known electrochemical couples. When only lithium is considered and oxygen is absorbed from the surrounding air environment, the battery has a specific energy of 11,972 Wh/kg or 11,238 Wh/kg if the reaction product is lithium peroxide (Li2O2) or lithium oxide (Li2O), respectively. With internally carried oxygen, the specific energy is still as high as 3,622 Wh/kg or 5,220 Wh/kg if the reaction product is lithium peroxide (Li2O2) or lithium oxide (Li2O), respectively. Even considering a more than 50% weight contribution from other inactive materials (including the air electrode, separator, electrolyte, and packaging), the specific energy of the lithium/air battery is still capable of reaching an order of magnitude larger than that of conventional lithium or lithium ion batteries.